1. Technical Field of the Invention
The present invention relates to a solid-state imaging device arranged with a plurality of unit pixels so that a signal can be desirably selected and read out of the individual unit pixel under address control, and to a control method for the same. More particularly, the invention relates to a solid-state imaging device of a type having unit pixels each structured by a photoelectric converter element and three transistors without having a select transistor, and to a drive control method for the same.
2. Description of the Related Art
The amplification-type solid-state imaging device (APS: Active Pixel Sensor, also called a gain cell) as one kind of X-Y addressing solid-state imaging device is structured with pixels by the use of active elements (MOS transistors) such as the MOS structure, in order to provide the pixel with an amplification function. Namely, the signal charge (photoelectrons), stored on the photodiode as a photoelectric converter element, is amplified by the active element and read out as image information.
In the X-Y addressing solid-state imaging device of this +kind, a pixel region is constituted, for example, by arranging a multiplicity of pixel transistors in a two-dimensional matrix form. The storage of a signal charge corresponding to incident light is commenced line by line (row) or pixel by pixel so that the current or voltage signal based on the stored signal charge can be sequentially read out of the pixels by address designation.
<Conventional Unit-Pixel Structure: First Example>
FIG. 19A shows a first example of conventional unit pixel 3. The unit pixel 3 of the first example is of a 4-transistor structure generally employed as a CMOS sensor, as broadly known in the conventional.
The unit pixel 3 of the first example has a charge generating section 32 having a photoelectric converting function to convert light into charge and also a charge storing function to store the charge, a read select transistor 34 as an example of a charge reading section (transfer gate section/read gate section) with respect to the charge generating section 32, a reset transistor 36 as an example of reset gate section, a vertical select transistor 40, and an amplifier transistor 42 in a source-follower configuration as an example of a detector element to detect a potential change at a floating diffusion 38.
The read select transistor 34 is to be driven by a transfer drive buffer 150 through a transfer line (read select line) 55. The reset transistor 36 is to be driven by a reset drive buffer 152 through a reset line 56. The vertical select transistor 40 is to be driven by a select drive buffer 154 through a vertical select line 52.
Meanwhile, the unit pixel 3 has a pixel-signal generating section 5 in an FDA (floating diffusion amp.) structure with a floating diffusion 38 as an example of charge injecting section having a function of charge storing part. The floating diffusion 38 is a diffusion layer having a parasitic capacitance.
The reset transistor 36 of the pixel-signal generating section 5 has a source connected to the floating diffusion 38, a drain connected to the power source VDD, and a gate (reset gate RG) to which a reset pulse RST is to be inputted from the reset drive buffer 152.
The vertical select transistor 40 has a drain connected to the power source VDD, a source connected to the drain of the amplifier transistor 42, and a gate (particularly referred to as a vertical select gate SELV) connected to the vertical select line 52. To the vertical select line 52, a vertical select signal is to be applied. The amplifier transistor 42 has a gate connected to the floating diffusion 38, a drain connected to the source of the vertical select transistor 40, and a source connected to the vertical signal line 53 through the pixel line 51.
In this arrangement, the floating diffusion 38 is connected to the gate of the amplifier transistor 42. Accordingly, the amplifier transistor 42 outputs a signal commensurate with the potential at the floating diffusion 38 (hereinafter referred to as an FD potential) onto the vertical signal line 53 through the pixel line 51. The reset transistor 36 resets the floating diffusion 38. The read select transistor (transfer transistor) 34 transfers the signal charge generated by the charge generating section 32 to the floating diffusion 38. A multiplicity of pixels are connected to the vertical signal line 53. In order to select a pixel, the selected pixel only is turned on at its vertical select transistor 40. Thereupon, the selected pixel only is connected to the vertical signal line 53, to output a signal of the selected pixel onto the vertical signal line 53.
In this manner, the unit pixel 3 is generally structured with the vertical select transistor 40 for the purpose of pixel selection. The unit pixel 3 in most today's CMOS sensors possesses a select transistor.
<Conventional Unit-Pixel Structure: Second Example>
Contrary to this, there is a proposal of a unit pixel 3 structured by a photoelectric converter element and three transistors as shown in FIG. 19B (hereinafter referred to as a second example of unit pixel 3), as a technology for reducing the pixel size by decreasing the area the transistor occupies within the unit pixel 3 (see Patent Document 1, for example).
[Patent Document 1]
Japanese Patent No. 2708455
The unit pixel 3 of the second example has a charge generating section 32 (e.g. photodiode) for generating signal charge commensurate with the light received by photoelectric conversion, an amplifier transistor 42 connected to the drain line (DRN) and for amplifying a signal voltage corresponding to the signal charge generated by the charge generating section 32, and a reset transistor 36 for resetting the charge generating section 32. Meanwhile, a read select transistor (transfer gate section) 34, to be scanned by a not-shown vertical shift register through the transfer line (TRF) 55, is provided between the charge generating section 32 and the gate of amplifier transistor 42.
The gate of the amplifier transistor 42 and the source of the reset transistor 36 are connected to the charge generating section 32 through the read select transistor 34 while the drain of the reset transistor 36 and the drain of the amplifier transistor 42 are connected to the drain line. Meanwhile, the source of the amplifier transistor 42 is connected to the vertical signal line 53. The read select transistor 34 is to be driven by a transfer drive buffer 150 through a transfer line 55. The reset transistor 36 is to be driven by a reset drive buffer 152 through the rest line 56. The transfer drive buffer 150 and the reset drive buffer 152 are both to operate on two values, i.e. reference voltage 0 V and power voltage. Particularly, the low level voltage to be supplied to the gate of the conventional-example read select transistor 34 in the pixel is 0 V.
In the unit pixel 3 of the second example, the floating diffusion 38 is connected to the gate of the amplifier transistor 42 similarly to the first example. Accordingly, the amplifier transistor 42 outputs to the vertical signal line 53 a signal commensurate with the potential at the floating diffusion 38.
The reset transistor 36 has a reset line (RST) 56 extending in the row direction and a drain line (DRN) 57 provided common between most of the pixels. The drain line (DRN) 57 is to be driven by a drain drive buffer (hereinafter referred to as a DRN drive buffer) 140. The reset transistor 36 is to be driven by the reset drive buffer 152, to control the potential at the floating diffusion 38. Here, the technique described in Patent Document 1 has the drain line 57 separated in the row direction. The drain line 57 has to flow a signal current in an amount of the pixels of one row, hence being actually a line common between all the rows in order to flow current in the column direction.
The signal charge generated by the charge generating section 32 (photoelectric converter element) is transferred to the floating diffusion 38 by the read select transistor 34.
Here, the unit pixel 3 of the second example is not provided with a vertical select transistor 40 connected series with the amplifier transistor 42, differently from the first example. Although the vertical signal line 53 is connected with a multiplicity of pixels, pixel selection is effected not by a select transistor but under control of FD potential. Usually, the FD potential is kept low. When selecting a pixel, the FD potential is raised high at a selected pixel, to send a signal of the selected pixel onto the vertical signal line 53. Thereafter, the FD potential at the selected pixel is returned to a low. This operation is effected simultaneously on the pixels of one row.
For FD potential control in this manner, operations are made, i.e. 1) when the FD potential on the selecting row is made high, the drain line 57 is raised to the high to thereby raise the FD potential to the high through the reset transistor 36 on the selected row, and 2) when the FD potential on the selected row is returned to the low, the drain line 57 is made low to thereby decrease the FD potential to the low through the reset transistor 36 on the selecting row.
However, the present inventor has trial-manufactured a solid-state imaging device structured by the second type of unit pixels 3, and thereby recognized the problems, i.e. 1) the characteristic is different between the pixel at the periphery and the pixel at the center, thus causing shading phenomenon wherein, particularly, the maximum charge (saturation electrons) to be stored on the photoelectric converter element is less at the center, and 2) dynamic range is narrow.
In respect of the two problems, the present inventor has analyzed the phenomena and revealed the followings.
1) The drain line 57 is a line extending through nearly the entire of the pixel region. When driving it, there occurs potential fluctuation on the well (hereinafter, explanation continued with a P-type well) of the pixel region. Although the contact for giving a potential to the P-well is placed around the pixel region, the manner of P-well potential fluctuation differs depending upon a distance from the contact, thus causing variations in the pixel characteristic. Particularly, when the drain line 57 is made low, the P-well is biased toward the negative, thereby leaking a signal charge from the charge generating section 32 to the floating diffusion 38 and P-well. At the center located distant from the contact of the P-well, the P-well potential fluctuation is greater to thereby lose saturation electrons at the center. This is called saturation shading.
2) Following the period of driving the selected-row pixel and reading a signal therefrom (H ineffective period), there is a period to sequentially output the signal to the outside (H effective time). During the H effective period, in the case of driving to maintain the drain line 57 high, the FD potential gradually rises due to the leak current through the reset transistor 36. Due to this, because the difference decreases between the selected row and the non-selected row, dynamic range is delimited and narrowed.
The two problems and analysis results are the new matters not to exist on the CMOS sensor of a type to select every pixel by the vertical select transistor 40.
The present invention has been made in view of the above situation, and it is a first object to provide a drive technique capable of improving the shading phenomenon, particularly the saturation shading phenomenon, as encountered in using a device having unit pixels in a three-transistor structure.
Meanwhile, the present invention has been made in view of the above situation, and it is a second object to provide a drive technique capable of improving the reduction of dynamic range resulting from the leak current from the reset transistor, as encountered in using a device having unit pixels in a three-transistor structure.